The hydroformylation of ethylenically unsaturated compounds to form aldehydes and/or alcohols is of considerable industrial importance. The process has been in commercial operation for decades and over the years much development work has been done to optimise the reaction conditions, the catalyst system and the equipment. Although significant progress towards higher yield and product selectivity has been made, further improvement of the process is still needed.
In the present applicant's WO 95/05354 there is disclosed a hydroformylation process whereby the catalyst system comprises a source of platinum group metal cations, preferably palladium or platinum cations, a source of anions other than halide anions, a source of bidentate ligands as defined below and a catalyst promoter comprising a source of halide anions. It is shown that the presence of the halide anion accounts for a considerably enhanced activity and selectivity of the process, i.e. suppression of paraffin make. However, the catalyst composition appeared to be sensitive to small variations of halide ion concentrations, the positive effect of which having a sharp peak at a molar ratio to the cation of about 0.4:1. In the subsequent present applicant's U.S. patent application Ser. No. 08/918.981, filed on Aug. 27, 1997, it is disclosed that the addition of water, in an amount of more than 0.6 wt % based on the total of the reaction mixture and up to its solubility limits under the reaction conditions, acts as a strong co-promoter with the halide anions.
In the aforementioned previous work, it was indicated with regard to the source of anions in the catalyst composition that any compound generating these anions may be used, preferably strong acids having a pKa value of less than 3 when measured in aqueous solutions at 18.degree. C. and most preferably derivatives of sulphonic acid such as methane sulphonic acid (pKa=-1.9), trifluoromethane-sulphonic acid (pKa=-5,7) and tert.butanesulphonic acid (pKa=-1.2).
The present inventors unexpectedly found that the catalyst compositions as described above generally suffer from the disadvantage that they are rather sensitive to even small amounts, from 0.005 wt % based on the total amount of the ethylenically unsaturated compounds in the feed upward, of diene and/or further multi-unsaturated alkenes in the reaction feed. The latter, in particular dienes, were found to form relatively stable complexes with the metal (cation) of the catalyst, thus hindering the progress of the reaction. In other words, dienes were found to be strong albeit reversible catalyst inhibitors. When the hydroformylation reaction is performed in batch operation, the presence of dienes and/or further multi-unsaturated alkenes in the feed results in a significant slowing of the reaction. When the hydroformylation reaction is performed in continuous operation, their presence in the feed may result in impractically low production rates.
It has now been found that by a careful selection of the anions in the catalyst compositions based on palladium, as they are more broadly defined in WO 95/05354, the hydroformylation reaction can proceed efficiently even when up to 10 wt % of the olefinically unsaturated compounds in the feed are dienes and/or further multiply unsaturated alkenes. According to the present invention the anions have a pKa of between -1 and 4.